In order to obtain accurate readings from a flowmeter, it must be calibrated from time to time by determining its characteristics, i.e., the constant of proportionality between the flow rate of the fluid flowing through the flowmeter and the response given by the flowmeter, sometimes called the K-factor of the flowmeter. In the case of a turbine type flowmeter that develops electrical oscillations proportional in number to the volume of flow through the flowmeter, this characteristic is expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing therethrough. The flowmeter characteristic is a function of the type of fluid, as well as the fluid temperature, pressure, and flow rate, and varies as the parts of the flowmeter wear in the course of use. Apparatus to determine the characteristic of a flowmeter while in an operating fluid system is called a prover. Apparatus to determine the characteristic of a flowmeter in a self-contained system, i.e., not in an operating fluid system, is called a calibrator.
U.S. Pat. No. 4,152,922 discloses a small-volume prover that employs mechanical volume displacement techniques. The prover has a measuring piston that travels through a measuring cylinder as a fluid barrier in synchronism with fluid passing through the operating fluid system that includes the flowmeter under test. A rod connects the measuring piston to a fluidically actuated control piston in a control cylinder which serves to hold the measuring piston at the upstream end of the measuring cylinder between test runs and return the measuring piston to the upstream end of the measuring cylinder after each test runs. When the measuring piston is released at the upstream end of the measuring cylinder to start a test run, the momentum of the fluid flowing through the system rapidly accelerates the measuring piston to the same speed as the fluid flowing through the measuring cylinder, which is representative of the flow rate passing through the flowmeter. The K-factor is determined by counting the number of pulses produced by the flowmeter during the time interval of a given volumetric displacement of the measuring piston.
U.S. Pat. No. 4,627,267 discloses a flowmeter calibrator which has a fluid line external of the measuring cylinder of the calibrator connected between the inlet and outlet thereof. A flowmeter is connected in the fluid line at a pressure null point so as to maintain the pressure of the fluid passing through the flowmeter at a predetermined constant value. A rod is connected to a measuring piston adapted to travel through the measuring cylinder as a fluid barrier. The rod drives the measuring piston through the measuring cylinder at a predetermined, constant speed and thereby determines the flow rate of the calibration. The displacement of the measuring piston is sensed as it travels through the measuring cylinder during a test run, while the pulses produced by the flowmeter are counted during the time interval in which the piston displaces a given volume. Because the calibrator has a small fixed volume, the pressure and temperature of the fluid passing through the flowmeter can be closely controlled, and any type of test fluid can be used irrespective of cost. Thus, the pressure, temperature, and type of test fluid can be selected so as to duplicate the actual conditions of the flowmeter in its operating system. Moreover, the flow rate can be closely controlled by regulating the driving speed of the rod.
In U.S. Pat. No. 4,627,267, the driving rod extends through both ends of the measuring cylinder, thereby adding to the overall size of the calibrator, and is pushed as it is driven during a test run.